Thank you very much, problem sorted out in a few minutes. The idea of starting again with FF 56 had occurred to me after I'd left my question, but I wouldn't have known where to find the setup program for it. So thanks again for your help, and especially for the link to the setup program!

Misinformation from the above poster. Yes it does work but....

Going back : unfortunately 56.0.2 is no longer safe to use for every day use do to secuity issues, but 52.5.0 ESR is. It will continue to get security updates until May 2018, and you can download and install it from this page: https://www.mozilla.org/en-US/firefox/organizations/all/ Note : Legacy Extensions will be deleted or removed in any version update after May 2018.

You should make a backup of your Profile before going back and just because: https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

Please let us know if this solved your issue or if need further assistance.

Great, thank you for your help.